F-m control track operation



retested Dec. 12, 1944,

F-M CONTROL TRACK OPERATION Edward W. Kellogg, Indianapolis, lind., assignor of Delaware to Radio Corporation of America, a corporation Application November ii, 19%, Serial No. 509,932

12 Cia.

This invention relates to record-responsive control systems, and more particularly to immovements in the art of automatically adjusting some factor, such as the gain of an amplifier, the physical position of a mechanical load .device, or the like, in accordance with variations in frequency of a recorded control tone.

Numerous applications have been found for control of electrical or mechanical devices by means of a recorded alternating current which varies either in amplitude or frequency, according to the system chosen, the record being employed with a suitable reproducing device and a circuit which responds to the change in recorded amplitude orfrequency. One of the best-known applications of this principle consists in utilization of such a recorded alternating current to vary the gain of amplifiers in a sound-reproducing system. A continuous alternating cur.- rent or tone is recorded on the same record with the sound to be reproduced, and reproduced from the record by a separate reproducing device from that employed for converting the sound record into electrical currents. The recorded tone, usually designated as the control tone," is rectified and thereby caused to provide a varying direct current voltage which alters the gain of the sound amplifier. The control tone system makes it possible to provide large changes in the level or loudness of the reproduced sound. With constant amplification, maximum and minimum limits are imposed by the characteristics of the sound record, usually a definite overload point on the one hand and on the other hand background noise which becomes conspicuous when the recorded levels are very low; With a control tone the sound may be so recorded thatit is at all times safely below the overload point and well above ground noise level, and the control tone can provide large changes in amplification to aflford the desired musical or other sound eflects.

Control tones as just described are ordinarily of two types. In the first the frequency remains constant and the amplitude is varied,-while in the second type the amplitude is constant and the frequency is varied. In the variable frequency system the reproduced tone is applied to a circuit or network, commonly called a discriminator circuit, which is composed of reactances or combinations of reactance and resistance, and which has the property of developing across its output terminals, a voltage which depends on the frequency of the input,

although the input itself may be of constant (on ire-ices) magnitude. This varying voltage may then be rectified and caused to change the bias on a variable gain tube or other device for changing amplification in the sound channel.

The variable frequency control tone system is somewhat more complicated than the variable amplitude system, butit has the advantages that the control tone is all times recorded at a moderately high level and this makes the system less sensitive to disturbances due to imperfections in the record. Moreover,-since it is a part of the system that the tone is to be maintained at constant amplitude up to the point of its application to the discriminator, it is common practice to employ an amplitude-limiting circuit, such as an amplifier tube which reaches a sharp overload point and is thus incapable of transmitting alternating voltages above a certain magnitude. Such a" limiter still further reduces the danger of changes in the final output of the control tone system due to record imperfections. The imperfections to which I refer are the same things which cause noise in reproduced sound. They maybe scratches, pits or dirt in the case of a groove record such as a phonoproduce voltages higher than those of the recorded tones, and these might produce considerable disturbance or changes 'in amplifier g'ain.

The limiter prevents the noise sources from ever than that of the recorded tone.

Control tones can be applied to many devices other than variable gain amplifiersfor example, to accomplish any actions for which a cam might be used in a mechanical system, 'Suppose, for

small, the duration of the cycle moderately short,

and the variety of actions not too large, the controls may be advantageously provided by mechanical cams; but where these limitations do not exist and greater freedom of performance is needed, the control tone system may be employed-using servo electric motors or other devices which can be remotely controlled.

Although the variable frequency control tone system as described above has the advantr gas which I have 'pointed out, it is subject to one serious disadvantage, since the discriminator responds to frequency, any deviation from correct operating speed, by the apparatus by which the tone is recorded or reproduced, results in a change of frequency from that which was intended. For example, it might be desired at a certain place on the record to record at 1000 cycles. We shall assume that the recordist correctly adjusts an oscillator to 1000 cycles, but if the recording machine is running 1% slow at the moment, the recorded waves will be slightly shortened, and if the reproducing machine runs at the correct speed the reproduced tone would be approximately 1010 cycles; and if the reproducing machine itself should run 1% fast, the

reproduced tone might be as high as 1020 cycles. Experience with sound recording. and reproducing machines indicates that random deviations of this magnitude are extremely likely to occur, as well as continuous or slowly drifting changes of speed. It is obvious that the discriminating circuit will respond to these incorrect frequencies just as if they were the correct or desired frequencies. For some applications there may be large enough tolerance for random variations in the final control voltage so that satisfactory performance can be obtained by using machines of reasonably good speed regulation and with no further insurance against impaired results due to speed changes.- For other applications the small variations due to speed changes may interfere with the desired overall results.

to those skilled in the art upon consideration of the following. description, with reference to the accompanying drawing, of which Figure 1 is a schematic block diagram of a control system embodying the instant invention, Figures 2 and 3 are graphs of the transmission characteristics of the discriminator circuits in the system of Figure 1, Figures 4 and 5 are schematic circuit diagrams of discriminators having the characteristics illus trated by Figures 2 and 3 respectively, and Figure 6 is a schematic circuit'diagram of an emplifier and limiter of the type employed in the system of Figure 1.

Ihe present invention contemplates the use of two tones recorded simultaneously on the same record carrier, one of constant amplitude but variable frequency, to perform the controlling operation, and the other constant in both frequency and amplitude for the purpose of neutralizing the effects of changes in the speed of operation of the recording or reproducing equipnient. The variable frequency tone is applied to a frequency responsive circuit, or discriminator, similar in function to those used commercially in F-M radio application and the like, which cause a change in the frequency of the input tone to give rise-to a change in the magnitude of 2. rec-. tified voltage. The constant frequency tone is applied to a second discriminator having a frequency response'characteristic which in the preferred embodiment of my invention) slopes inthe opposite direction to that of the first, and the rectified outputs of the two discriminators are 'aecarea combined in such proportions that the effects of speed changes are balanced against each other for minimum total response to speed variations.

Referring to Figure 1, a reproducer system l is provided for producing a combined output voltage which comprises a nominally constant frequency, constant amplitude component, herein referred to as a pilot tone, and a constnant amplitude control tone component which varies in frequency, having been recorded according to a predetermined schedule or time function, for the purpose of accomplishing some desired control. The reproducer 1 may comprise a film phonograp producing photographically recorded tones, or a disc phonograph, magnetic tape device, or any known mechanism capable of providing A. C. electrical output in response to and corresponding to a suitable record. Inasmuch as numerous such systems are well known to those skilled in the art, it is deemed unnecessary to describe the reproducer i in detail. For the purpose of describing the present invention, it is suficient to state that the record elements to be employed with the reproducer i carry a composite wave comprising the control and pilot tones, or else two tones recorded side by side, so that each provides its respective input into the reproducing device i.

The output circuit of the reproducer i is connected to a control tone channel 3 and to a pilot tone channel 5. The control channel 3 includes a high pass filter 1, through which the output v of the reproducer l is applied to an amplifier and limiter 9. The output circuit of the amplifier 0 is connected toa discriminator H, which is in turn connected to the input circuit of a rectifier 13. The output circuit of the rectifier l3 includes a voltage divider I5 provided with a manually adjustable tap II. The filter I is designed to pass frequencies of, for example, 1000 cycles per second and higher, and the discriminator II is designed toprovide, in response to constant amplitude input, an output having an amplitude which is a linear function of the logarithm of the input frequency. Such a characteristic is illustrated by the curve a of Figure 2, which for purposes of illustration is here shown as usable throughout the range 1000 to 2000 cycles.

The pilot channel 5 is similar to the control channel 3, but includes a low pass or band pass filter l9 instead of the high pass filter i of the control channel. The filter I9 is designed to pass, for example, 500 cycles per second, and to reject the control tone frequencies of 1000 cycles per second and higher. The amplifier and limiter in the opposite direction from that of the control channel discriminatonin the neighborhood of the pilot tone frequency, 1. e., 500 cycles per second The characteristic of the discriminator 23 need not be linear, since it will be required to operate only over a narrow, band width corresponding to the frequency variations due to the speed variation which will occur' in the operation of the recording or reproducing system. A typical frequency response curve of a discriminator suitable for this purpose is illustrated by the curve of Figure 3. The rectifier 25 and the voltage divider 21 may be identical with the correspondthis being that such an average rectified voltage is less afiected by accidental defects in the record such as scratches or dirt.

The adjustable taps l1 and 29 of the voltage dividers l and 21 are connected through resistors 3| and 33 to a junction point 35, which is connected to the input circuit of a D.-C. magnitude-responsive control device 31. Included in the connection between the point 35 and the device 31 is an adjustable D.-C. bias source, comprising a battery 39 and a voltage divider 4|. The control device 3? may be an amplifier, a servomotor, or any device which is to be controlled in accordance with the record associated with the reproducer i.

The operation of the above described system is asfollows:

The taps IT. and 29 are adjusted on the voltage dividers i5 and 2'! respectively to provide equal frequency discrimination sensitivities in the control and pilot channels, i. e., equal percentage frequency changes produce equal output voltage changes at the taps ii and 29. For example, if a 1% increase in the frequency of the control tone, say from 1500 to 1515 cycles, causes a decrease of 2 volts in the rectified voltage applied to divider i5, and a 1% increase in the frequency of the pilot tone, or from 500 to 50 cycles produces an increase of 1.8 volts in the voltage impressed across divider 21, the sliding contacts I1 and 29 would be adjusted to equalize these two effects at junction 35in which case a 1% increase in reproducing speed of the record would have no effect on the rectified voltage applied to control circuit 31, the one being neutralized by the other.

This adjustment may be accomplished in several ways; for example, a calibration record carrying 500 and 1400 cycle tones, synchronously frequency modulated at one cycle per second over a band width of, say, two percent, is placed in the reproducer i. The adjustment for correct neutralization of the efiect of a speed change may also readily be made using any good recording of pilot tone and a constant frequency tone within the control tone operating range, the speed of the reproducing machine being caused .to execute a cyclic change, as by unbalancing a fiywheel. A voltmeter is connected from the junction point 35 to ground. One or both of the taps l1 and 29 may be adjusted so that the voltmeter reading remains constant. It will be found that for substantially every position of the tap ll,

there is a correct adjustment for the tap 29, and

vice versa. Which pair of positions is selected will affect the overall sensitivity of the system, like a gain control in the circuit of the reproducer i.

After said adjustment is properly made, a rec -ord bearing a consant frequency pilot tone and respectively. Any variations in amplitude are removed by limiting action in the amplifiers, as explained more fully herein below. The output These comof the amplifier 9 comprises a constant amplitude voltage, varying in frequency within the range of 1000-2000 cycles per second, in accordance with the record. Superimposed upon said frequency variation is a further frequency variation caused by substantially unavoidable changes in the speed of operation of the reproducer i.

The discriminator ll, being unable to distinguish between modulation on the record and modulation caused by variation in record speed, provides an output corresponding to the sum of these two modulations. This output is rectified by the rectifier l3. providing a D.-'C. output having instantaneous magnitude proportional to the amplitude of the discriminator output. v

The pilot channel 5 operates in the same manner as the control channel 3. with the exceptions that the input to the discriminator 23 is nominally of constant frequency, and therefore is modulated only by variations in record speed, and the frequency characteristic of the pilot discriminator 23 slopes oppositely to that of the discriminator II. The output of the pilot channel rectifler 25 thus varies in magnitude in accordance with the record speed variations, but in the opposite sense from that of the control channel.

The two outputs are combined at the junction point 35. The resistors 3i and 33 are preferably each made of much higher resistance than the voltage dividers or potentiometers l5, ZLand also of high resistance compared with the input circuit resistance of the device 31 for these conditions give awider range of voltage control and more uniform relation between slider position and voltage. The voltage at said input circuit is then substantially proportional to the sum of the voltages at the taps l7 and 29. Variations in record Spe d produce little or no effect on the utilization The control sensitivity of the system, in volts per octave, may be regulated by simultaneous or coordinated adjustment of the voltage dividers i5 and 21, or better by means of a voltage divider (not shown) in the output circuit of the reproducer I or in the input circuit of the control circuit 31. The position of the range of operation, or zero setting, may be adjusted by means of the voltage divider 4| to control the D.-C. bias upon which the control voltage is superimposed. It is obvious that with the potentiometers l5 and 21 adjusted for correct neutralization of speed changes, the relative'levels of the pilot and con trol tones must remain constant. The limiters can take care of some variations, and with the balancing adjustments at a given setting the limiters should also remain adjusted to control the tone levels at fixed relative values prior to their application to their respective discriminators. In order not to depend too much on the limiters the recordings should all be made with a standardized ratio of control tone to pilot tone amplitude.

Referring to Figure 4. the circuit of the discriminator ll of the control channel includes a parallel-T network 43 and a conventional lowpass network 45 connected in cascade. The parallel T 43 comprises a low-pass section formed by series resistors 41 and 43 and a shunt capacitor 5|, and a high-pass section in parallel therewith, comprising series capacitors 53 and 55, and a shunt resistor 51. The values of these elements are interrelated, in accordance with well known practice, to cause cancellation of the outputs of the two parallel channels, and hence infinite attenuation, at a frequency of approximately 2200 cycles per second. It is characteristic of this typeof circuit to provide transmission varying substantially linearly with respect to the logarithm of the frequency over a substantial range of frequencies below the null, or cancellation frequency, providing the input and load resistances properly selected. These conditions are met by determination of the values of resistors 59 and BI respectively. The characteristic of the parallel T circuit has two limbs, a and b (Fig.2) The transmission represented by limb b is undesirable, since harmonics generated by the limiting amplifier 9, if allowed to pass to the rectifier 93, would cause improper operation. Accordingly, the low pass section 45, designed to cut off slightly above 2200 cycles, is provided. The transmission characteristic of this section is illustrated by the curve,c of Figure 2. The resultant transmission of sections 43 and 45 together is substantially the same as the line a of channel may be much simpler than that of the control discriminator H, since it is required to operate only over a relatively narrow band width.

Refer to Figure 5. A parallel resonant circuit comprising an inductor 63 and a capacitor 65 is connected as a shunt element in a T network, wit hseries resistors 57 and B9. The LC product of the elements 63 and 65 is determined so that resonance occurs at some frequency above the pilot tone frequency, say at 600 cycles per second. The Q of. this resonant circuit, and .the values of the resistors 6'! and 69 will affect the slope of the transmission characteristic. Since the overall response of the pilot channel isreadily adjustable by means of the voltage divider 27, it is evident that there is considerable flexibility in the design of this discriminator circuit. It is in general desirable to tune it to such a frequency that the mean pilot tone frequency is on the straightest part, which is also approximately the I steepest part of the characteristic, as shown in tive discriminators and rectifiers, and then'combine the rectified voltages, as already described, in appropriate proportions to neutralize the effects of speed changes:

Referring to Figure 6, the limiting action is provided'by two devices, each well known to the art, the one an automatic volume control which alters the gain of an amplifier stage as required to reduce variations, and thesecond a limiter of the overload type, which virtually short circuits all voltages in excess of a predetermined magnitude. These devices supplement each other, to give more nearly complete amplitude control. The .amplitude control circuits 9 and 2| each include electron-discharge tubes H and i3, coupled in cascade in conventional manner by a capacitor J5 and resistors 11 and 79. The anode circuit of the tube '73 includes transformers 8| and 83. The transformer 8| is connected to a full waverectifier comprising diodes 85 and 87 and a resistance-capacitance filter 89. The negative output terminal of this rectifier is connected to the grid return of the tube ll, and the positive rectifier terminal is connected to a source of fixed grid bias C.

The transformer 83 is provided with two'closely coupled secondary windings 9i and 93. The winding 9| comprises the output circuit of the limiter-amplifier. The winding 93 is connected to a full wave biassed rectifier type limiter comprising diodes 95 and 97 and a battery 99.

The automatic volume control portion of Figure 6 operates as follows:

The tubes TH and it constitute and function as a conventional two stage amplifier, tube ll being of the variagle gain type. A portion of the output voltage of the tube i3 is impressed through the transformer 8i 0n the rectifier 85,91, providing a unidirectional output voltage substantially proportional to the output of the tube 53. The rectifier output is smoothed by the filter 89 and applied in series with the fixed bias 0- to the grid return circuit of the tube l I The polarity of this voltage is such as to aid the bias, so that as the output of the tube l3 increases, the negative bias on the grid of the tube ii increases, decreasing the gain aflorded by the tube H and thereby tending tomaintain the output of the tube H3 at a more or less constant level in spite of variations in the amplitude of the input to the tube it. v

The output of the tube i9 is coupled to the output circuit of the amplifier through the transformer 83; however, the limiter diodes 95 and 971 are effectively shunted across this output circuit by virture of the close coupling between the secondaries 9i and 99. So long as the output voltage across the diodes 95 and 97? does not exceed the voltage of the battery 99, the diodes do not conage.

duct. When the negative peaks tend to exceed the battery voltage, one diode, for example the diode, 95, will conduct, eflectively short circuiting the output and bringing the voltage down to a peak value substantially equal to the battery volt- Similarly, when the positive output peaks tend to exceed the battery voltage, limiting is provided by the diode 9?. Partial limiting is thus provided by the variable bias circuit including the rectifier 8 5, 8?, and the limiting is completed by the diodes and 9?. As a simplification it would be possibleto derive a bias-changing voltage from the currents flowing through thepeak limiting'diodes 95, 9'27. This arrangement is not illustrated, being only a variation of the general system described, wherein a separate rectifier 95, 87 furnishes the voltage for changing the grid bias of tube ii.

The invention has been described as a frequency responsive control system for use with a recorded variable frequency control tone. Undesired frequency variations, superimposed on the intentional variations, by changes in speed of the record element during recording and/or reproducing, are neutralized by recording a constant frequency pilot tone simultaneously with the control tone. The reproducedtones are separated by filters, separately. amplified and limited, and applied to discriminators having oppositely sloping frequency characteristics. The discriminator outputs are rectified and balanced against each other, providing a resultant D. C. output whose amplitude is a function substantially only of the ratio of frequencies of the reproduced control and pilot tones, rather than the absolute frequency of the reproduced pilot tone, over the range of speed variation which will occur in recording and reproducing. Since for small changes in speed, this ratio is independent of the actual frequencies involved, proportional variations in frequency caused by speed variations have no effect on the resultant ouput.

It is obvious that in order to secure the benefits of this invention with respect to imperfections in the speed constancy of the recording machine, it is necessary that the pilot tone and the com trol tone be recorded simultaneously, as will be the case if the, reco'rdist uses an oscillator of fixed frequency, say 500 cycles, and mixes its output with that of a variable frequency oscillator, say 1000 to 2000 cycles, the combined voltage being recorded through a single recording device. The desired result would not be obtained. if the pilot tone and control tone were recorded side by side in two separate operations, or by two successive exposures, wherein the record is run twice through a recording machine.

The system has been described as calling for discriminator circuits which have operating characteristics with opposite slopes. The invention is not limited to this characteristic, for similar results might be obtained with discriminator circuits for the control tone and pilot tone having the same direction of slope but with the final rectified voltages so combined that the output of the controlling device is afiected by the difference between the two rectifiedvoltages instead of by their sum. It should be recognized that in a system arranged as just described, the slopes may still be described as opposite, with reference to the final combined voltage, for an increase in the frequency of one tone will cause this final voltage to fall, while an increase in the frequency of the other tone will cause the:-

combined voltage to rise. If the system is arranged to work on the difference between'the .ever, may be employed and the benefits obtained by employing two variable tones,-. the pitches of both or which are varied inthe original recording in opposite directions and both of which contribute to the desired control. Both tones would be applied to discriminator networks and the rectified outputs combined to provide a voltage affected by the ratio of the recorded frequencies but wherein the efiects of a simultaneous rise or fall in both frequencies by the same percentage will be neutralized.

Throughout this specification and in the claims the word tone is used to designate an alternating current since such control systems gener- .on a record cooperating with said reproducing.

means, and varies further in frequency owing to variations in the speed of operation of said reproducing means, means for producing a second voltage of nominally constant frequency which varies in frequency only in proportion to said variation in speed, two frequency discriminator circuits having oppositely sloping transmission characteristics, means for applying one of said voltages to one of said discriminator circuits and the other of said voltages to the other of said discriminator circuits, two rectifiers connected respectively to said discriminator circuits, and means for adding the outputs of said rectifiers.

2. The method of producing a D. C. voltage which varies'in magnitude in proportion to the ratio of the frequencies of two components of a complex A. 0. wave, said components lying with in different frequency bands, comprising the steps of separating said components according to the frequency bands within which they lie, amplifying and limiting said components, varying the amplitude of one of said limited components as a linear function of the logarithm of the fre quency of said component, varying the amplitude of the other of said limited components as a predetermined function of frequency, separately rectifying said amplitude-varied components, and combining the products of said separate rectifications.

3. The method of producing a D. C. voltage I which varies in magnitude substantially in proportion to the ratio of the frequencies of two A. C. waves recorded on a. single record carrier, comprising the steps of separating said waves, varying the amplitude of the first of said waves as a. function of the frequency of said wave, varying the amplitude of the, second of said components as a second function of frequency, rectifying said first and second components, and

combining Said first and second rectified components.

4. A limiting amplifier including electron discharge means having an input circuit and. an

output circuit, and connected to provide ampli fication of voltage applied to said input circuit, rectifier means coupled to said output circuit to rectify a portion of the output voltage of said electron discharge means, filter means connected to said rectifier means, ans for applying the output of said filter me s to said input circuit as bias whereby the amplification afforded by said electron discharge means is controlled inversely in accordance with said output voltage, second rectifier means coupled to said output circuit, and

connected to a substantially constant potential D. C. source, whereby said output circuit is effectively short circuited with respect to output voltages exceeding a predetermined amplitude.

5. A record responsive controlrsystem including record reproducer means, filters providing mutu ally exclusive transmission characteristics connected to said reproducer means, limiting amplifiers connected respectively to said filters, discrlminators connected respectively to said limiting amplifiers, said discriminators providing o ppositely sloping characteristics of transmission vs. frequency, rectifiers connected respectively to said discriminators, adjustable voltage dividers connected to said rectifiers, and a voltage combining circuit connected to said voltage dividers, whereby similar coincidental variations in the frequencies of the outputs of said filters caused by variations in the speed of operation of said reproducer means are neutralized.

6.. In a control system wherein a recorded tone of variable frequency is employed to produce a varying rectified voltage, a record carrying two tones, means responsive to a change in the frequency of one of said tones for producing a rectified voltage, means responsive to a change in the frequency of the second of said tones for producing a second rectified voltage, and means for combining said voltages in such sense and in such relative magnitudes that when the frequencies both increase or both decrease by equal percentages, the net change in the combined voltage is substantially cancelled.

7. In a control system, a record carrying two tones, means for simultaneously reproducing both tones, a frequency responsive discriminator circuit to which the first of said tones is applied, a frequency responsive discriminator circuit to which the second, of said tones is applied, and means for. combining voltages derived from. said two tones.

8. In a record responsive control system inaeearae steps of separating said components according to the frequency bands within which they lie, amplifying and limiting said components, varying the amplitude of one of said limited components as a linear function of the logarithm of the frequency of said component, said linear function being predetermined so that said amplitude is decreased with increase in frequency,

varying the amplitude of the other of said limited components as a predetermined function of frequency so that said amplitude of said other component is increased with increase in frequency, separately rectifying said amplitudevaried components, and combining the products of said separate rectifications.

11. The method of producing a D. C. voltage which varies in magnitudesubstantially in proportion to the ratio of the frequencies of two A. C. waves recorded on a single record carrier, comprising the steps of separating said waves, varying the amplitude of the first of said waves as a function of the frequency of said wave, said function being predetermined so that said amplitude is decreased with increase in frequency, varying the amplitude of the second of said components as-a second function of frequency, said voltage of nominally constant frequency which varies in frequency only in proportion to said variations in speed, two frequency discriminator circuits, means for applying one of said voltages to one of said discriminator circuits and the other of said voltages to the other of said discriminator circuits, two rectifiers connected respectively to said discriminator circuits, and means for adding the outputs of said rectifiers.

9. The method of producing a D. C. voltage which varies in magnitude in proportion to the ratio of the frequencies of two A. C. voltages, comprised of varying frequencies and lying within different frequency bands, comprising the steps of separately amplifying and limiting said second function being predetermined so that said amplitude of said second component is increased with increase in frequency,- rectifying said first and second components, and combining said first and second rectified components 12. A limiting amplifier including electron discharge means having an input circuit and an output circuit, and connected to provide am= plification of voltage applied to said input cir-= cuit, unilaterally conductive means coupled to said output circuit through a D, C. source where= by output voltages in excess of a predcteed magnitude are short circuite'd, and means for varying said amplification in response to rec tifiedoutput of said electron discharge means, 

